Hi Preethi -
Yes, I'm also facing the same issue with report builder scheduler and Scheduled Reports Manager (.PDF reports). I would suggest, raise a ClientCare ticket for this issue.
Also, please check with your internal IT Team if they have made some changes internally. Following is the document which includes the IPS which needs to be white listed at your end. You can use this list of ranges to alter your organization's firewall settings to allow log-in access
https://helpx.adobe.com/analytics/kb/adobe-ip-addresses.html
Best,
-Suhail